THE MOLLUSCA 31 
derm, and become specialised to form the internal lining of the 
circulatory cavity in particular, and may even fill almost entirely 
the remainder of the blastocoele in the form of a false mesenchyme 
(caenogenetic or secondary mesenchyme), which constitutes the 
connective tissue. This naturally restricts the extension of the 
coelom properly so called, so that it is commonly reduced to the 
pericardial cavity. Since the mesodermic tissue gives rise, in this 
manner, to the epithelial wall of the coelomic cavity, to the lining 
of the circulatory cavity, and to the conjunctive tissue filling up 
the spaces between the organs, one must recognise it as sharing in 
the evolution, firstly, of the coelom and the excretory and repro- 
ductive organs derived from the coelom ; secondly, of the circulatory 
apparatus—the heart, ete. 
The coelom, of which the formation has been described above, 
is essentially a cavity communicating with the exterior, and its 
epithelial wall may be differentiated in two special ways—into 
excretory or renal elements, and into reproductive, and therefore 
caducous elements. In the most primitive process the kidneys are 
formed in connection with a portion of the coelom, with which 
they remain in complete continuity (Paludina). In other cases 
they are formed by a hollowing out of a portion of the mesoderm 
in contact with the pericardium (Bithynia, Limax, Cyclus, Dreissensia, 
etc.), or they may be formed independently in their definitive 
position (Cephalopoda). Eventually each kidney acquires a com- 
munication with the pericardium, and in all cases makes a connection 
with the exterior by an ectodermic invagination. The genital 
organs or gonads originate either from the wall of the coelom or 
pericardium (Paludina, Dreissensia), or in contact with the coelomic 
wall (Cyclas), or from a rudiment common to themselves, the 
pericardium and the kidney, or, finally, from distinct mesodermic 
elements. The continuity of the pericardium and gonads is 
well preserved only in the Aplacophora (Fig. 30, C) and adult 
Cephalopoda (Fig. 252, coe) ; in all other Molluses the genital organs 
are separated from the pericardial cavity and acquire communica- 
tions either with the kidneys or directly with the exterior. In the 
latter case the terminal portions of the gonaducts, together with 
the accessory genital glands, are ectodermic in origin. 
The heart may arise from a portion of the wall of the peri- 
cardium itself (Paludina), or a common rudiment may give rise to 
the wall of the pericardium and the heart (Pulmonata, Cyclas, 
Dreissensia; ete.), and in the latter case the origin of the heart may 
be paired (Cyclas, Cephalopoda) like that of the pericardium itself. 
The larvae of such Molluses as lay their eggs singly and free in 
the sea are hatched out very rapidly ; a few hours suffice in the 
case of Dentalium among the Scaphopoda ; twenty hours in 7vochus 
among the Aspidobranchs ; fourteen hours in Yoldia among the 
